Pollution control devices are employed on motor vehicles to control atmospheric pollution. Such devices include catalytic converters and diesel particulate filters or traps. Catalytic converters typically contain a ceramic monolithic structure which supports the catalyst. The monolithic structure may also be made of metal. Diesel particulate filters or traps are wall flow filters which have honeycombed monolithic structures typically made from porous crystalline ceramic materials.
Each of these devices has a metal housing (typically stainless steel) which holds a monolithic structure made of ceramic or metal such as steel. The monolithic structures have walls with a catalyst thereon. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in engine exhaust gases to control atmospheric pollution.
Ceramic monoliths are often described by their wall thickness and the number of openings or cells per square inch (cpsi). In the early 1970s, monoliths with a wall thickness of 12 mils and a cell density of 300 cpsi were common ("12/300 monoliths"). As emission laws become more stringent, wall thicknesses have decreased as a way of increasing geometric surface area, decreasing heat capacity and decreasing pressure drop of the monolith. The standard has progressed to 6/400monoliths.
With their thin walls, ceramic monolithic structures are fragile and susceptible to vibration or shock damage and breakage. The damaging forces may come from rough handling or dropping during engine assembly, from engine vibration or from travel over rough roads. The monoliths are also subject to damage due to high thermal shock, such as from contact with road spray.
The ceramic monoliths have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. For instance, the gap between the peripheral wall of the metal housing and the monolith may start at about 4 mm, and may increase a total of about 0.33 mm as the engine heats the catalytic converter monolithic element from 25.degree. C. to a maximum operating temperature of about 900.degree. C. At the same time, the metallic housing increases from a temperature of about 25.degree. C. to about 530.degree. C. Even though the metallic housing undergoes a smaller temperature change, the higher coefficiential thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling typically occurs hundreds or thousands of times during the life of the vehicle.
To avoid damage to the ceramic monoliths from road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monoliths and the metal housings (thereby bypassing the catalyst), mounting mats or mounting paste materials are disposed between the ceramic monoliths and the metal housings. The process of placing the monolith within the housing is also called canning and includes such steps as wrapping a sheet of mat material around the monolith, inserting the wrapped monolith into the housing, pressing the housing closed, and welding flanges along the lateral edges of the housing. The paste may be injected into the gap between the monolith and the metal housing, perhaps as a step in the canning process.
Typically, the paste or sheet mounting materials include inorganic binders, inorganic fibers, intumescent materials, organic binders, fillers and other adjuvants. The materials may be used as sheets, mats, or pastes. Known mat materials, pastes, and intumescent sheet materials used for mounting a monolith in a housing are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.), U.S. Pat. No. 4,385,135 (Langer et al.), U.S. Pat. No. 5,254,410 (Langer et al.), U.S. Pat. No. 5,242,871 (Hashimoto et al.), U.S. Pat. No. 3,001,571 (Hatch), U.S. Pat. No. 5,385,873 (MacNeil), U.S. Pat. No. 5,207,989 (MacNeil), and GB 1,522,646 (Wood). With any of these materials, the mounting material should remain very resilient at a full range of operating temperatures over a prolonged period of use.
To continually improve emission standards, it has been desired to move the catalytic converter closer to the engine and thereby increase the temperature of the exhaust gasses traveling through the catalytic converter. The hotter catalytic converter and exhaust gasses therein increase the efficiency of the reactions which remove pollution from the exhaust gasses. As hotter catalytic converter temperatures are used, the mounting materials must be able to withstand the severe temperatures. In addition, the thermal transmission properties of the mounting material become more important toward protecting closely mounted engine components from the hot exhaust temperatures. Decreasing the converter skin temperature is important in preventing heat damage in the engine compartment and radiation into the passenger compartment.
It has also been desired to continually decrease wall thicknesses of the ceramic monolithic structure to enhance the catalytic converter operation. Extremely thin wall monoliths, such as 4/400, 4/600, 4/900, 3/600, 3/900 and 2/900 monoliths, have been developed or are expected to be developed in the not too distant future. The monoliths with extremely thin walls are even more delicate and susceptible to breakage. Typical intumescent mounting structures provide compression pressures which increase during use of the catalytic converter to a pressure above the initial mounting pressure. Increasing compression pressures during use of the catalytic converter also reduce the ability of support mats or pastes to sufficiently insulate the monolith from vibration damage or mechanical shock. Because of these various problems, published reports have advised against using intumescent mounting mats for extremely thin wall monoliths mounted close to the engine. See for example Umehara et al., "Design Development of High Temperature Manifold Converter Using Thin Wall Ceramic Substrate", SAE paper no. 971030, pg. 123-129, 1997.
The exposed edges of the mounting materials are subject to erosion from the pulsating hot exhaust gases, particularly as the mounting materials are thermally cycled numerous times. Under severe conditions, over a period of time, the mounting materials can erode and portions of the materials can be blown out. In time, a sufficient amount of the mounting materials can be blown out and the mounting materials can fail to provide the needed protection to the monolith.
Solutions to the erosion problem include the use of a stainless steel wire screen (see e.g., U.S. Pat. No. 5,008,086 (Merry)) and braided or rope-like ceramic (i.e., glass, crystalline ceramic, or glass-ceramic) fiber braiding or metal wire material (see, e.g., U.S. Pat. No. 4,156,333 (Close et al.)), and edge protectants formed from compositions having glass particles (see, e.g., EP 639701 A1 (Howorth et al.), EP 639702 A1 (Howorth et al.), and EP 639700 A1 (Stroom et al.)) to protect the edge of the intumescent mat from erosion by exhaust gases. These solutions employ the use of state of the art mounting materials as the primary support for the monolith.
Known bonded multilayer mounting mats are typically made by first separately forming the layers and then bonding the layers together using an adhesive or a film or other means such as, for example, stitches or staples. Typically, adhesively or film bonded multilayer mounting mats contain higher levels of organic material which produces undesirable smoke and odor when used in a catalytic converter. To prevent such smoke and odor, the mounting mats would have to be preheated before installation to bum off the organic bonding materials. The adhesive or film bonding layer also affects the thermal properties of the mat. Additionally, such mounting mats are more expensive to manufacture due to the cost of bonding the layers together and the cost of the adhesive or film used. Some disadvantages of mechanically bonded or attached multilayered mounting mats include the expense of added steps and materials and the mat may be weakened at the point of mechanical attachment such as where stitches or staples perforate the mat. Other multilayer mounting mats are comprised of separate layers that must be individually mounted within the catalytic converter housing.
A disadvantage of a single layer mat or sheet containing expandable graphite or a mixture of expandable graphite and unexpanded vermiculite is that typically such single sheet constructions having a homogeneous or uniform composition throughout the sheet require relatively high amounts of expandable graphite for the desired low temperature expansion which increases the cost of the mat.
A need thus exists for a mounting system which is sufficiently resilient and compressible to accommodate the changing gap between the monolith and the metal housing over a wide range of operating temperatures and a large number of thermal cycles. While the state of the art mounting materials have their own utilities and advantages, there remains an ongoing need to improve mounting materials for use in pollution control devices. Additionally, one of the primary concerns in forming the mounting mat is balancing between the cost of the materials and performance attributes. It is desirable to provide such a high quality mounting system at the lowest possible cost.